Blood platelet functional activities include aggregation, contraction and secretion - all thought to be contractile or motile in nature, and to be dependent on subcellular fibers. Various aspects of platelet motile functions have been attributed to microfilaments (5-7 nm, actin) and microtubules (25 nm, tubulin). A third subcellular fiber - intermediate filaments has been identified and isolated by the applicant from human blood platelets; these intermediate-type filaments, distinct from actin, myosin an tubulin, are thought to have various cytoskeletal or motile functions and to be functionally linked to microtubules in other cells. The proposed study is a systematic characterization of the intermediate filaments and other microtubule associated (nontubulin) proteins in platelets. The objectives of the proposed studies are to characterize platelet intermediate filaments; detailed biochemical studies involve purification of the constituent polypeptide, studies of its amino acid composition and comparison of the antigenic relationship of platelet intermediate filaments to those of other types of human cells. Studies will also establish the location of intermediate filaments in subcellular fractions of platelets (granules, surface membranes) and with microtubules using a specific antibody prepared against the polypeptide constituent. Studies to demonstrate the presence and function of other microtubule associated proteins in platelets will be performed by employing a microtubule polymerization assay; the platelet proteins which function to promote microtubule assembly and become associated with polymerized microtubules will be identified and characterized. The polypeptide composition of microtubules isolated as intact structures to ultrastructural homogeneity will also be analyzed.